At the beginning of the season both Liverpool and Manchester United were undergoing periods of transition, though of vastly differing sorts.
Brendan Rodgers' side faced the challenge of replacing the pace, verve and goals of the departed Luis Suarez, while United had a new incoming manager and philosophy to implement. It took both teams a while to adapt to the circumstances, but they have come out the other side stronger for it.
They face off at Anfield on Sunday as two of the Premier League's most in-form teams. Liverpool have won five on the bounce as they march towards consecutive top-four finishes, while United have taken 19 points from the last available 24 and also look like they have a chance of making it back into Europe's elite competition. Sunday's result will have a huge impact on the outcome of the race for a Champions League place.
During those summer months of preparation at England's most successful clubs, there were obvious choices as to who exactly would prove most important for their respective teams. Daniel Sturridge and Raheem Sterling were the obvious choices for Liverpool, while Wayne Rooney and David De Gea were most likely to step up for United. While each has been key in their side's recent resurgence, there have been less celebrated players at the heart of it all.
It is no coincidence the best performance of Manchester United's season came when an out-of-sorts Angel Di Maria missed their 3-0 win over Tottenham through suspension and Michael Carrick made his return to central midfield to shine. Jordan Henderson, meanwhile, has grown into his role as Liverpool's leader.
Louis van Gaal has struggled to cope without Carrick for much of the season and will be glad to have him back. The midfielder scored one and set up another in the 3-0 win over Spurs last weekend, recycling possession in a rapid manner that United's other players hadn't managed previously this season.
Carrick keeps the ball moving quickly and played a fine pass for Marouane Fellaini to open the scoring early on against Mauricio Pochettino's men.
They are more fluent with Carrick at the base of midfield -- his 66 passes per start is among the highest tallies in the Premier League, with only Yaya Toure, Cesc Fabregas, Aaron Ramsey and Fernandinho seeing more of the ball. He protects the defence decently enough, making a relatively modest 1.7 tackles and 1.5 interceptions per game, but the fact that United are so much more effective with the ball, keeping it higher up the pitch and doing more with it, means that chances are harder to come by for the opposition.
United have thus conceded just seven goals in the 13 Premier League games Carrick has started this season (0.54 per game), while they have shipped 19 in 16 games without him (1.19 per game). Their win rate of 69.2 percent when he starts drops to 43.8 percent without him. Clearly he is critical to Van Gaal's team.
He will come face to face on Sunday with Liverpool's new captain, Henderson, who has made Steven Gerrard's rapid decline over the past 12 months all the more bearable with increasingly mature displays.
When wearing the armband, Henderson has led Liverpool to eight wins and a draw in nine and Liverpool have scored 16 goals, conceding just twice. This has coincided with Liverpool finding their feet in their 3-4-3 formation, but Henderson has led by example in that time nonetheless.
Providing a driving force from midfield, his performances have both attacking and defensive input, with his goal in the 1-0 win at Swansea his third in his last three. He now has five goals for the season and seven assists to his name, all of which have come in open play, ranking him third in that regard behind only Fabregas (nine) and Gylfi Sigurdsson (eight).
With a WhoScored rating of 7.26 he is among the top-performing central midfielders in England and he has been duly rewarded with another call-up to the national squad. None other than the returning Carrick will join him when Hodgson's men meet up next week.
With him back in the United and England fold, both teams are look significantly stronger in central midfield and there is good reason to believe that the metronomic Carrick and tireless Henderson would complement each other wonderfully.
For this week, however, they remain rivals and will both have a big part to play in what is a huge game in the race for the top four at Anfield on Sunday.
All statistics courtesy of WhoScored.com, where you can find yet more stats, including live in-game data and unique player and team ratings.